With recent development of portable equipment, an increasing number of batteries have been used and the reduction in price of primary batteries as well as secondary batteries has been strongly required by the market.
One proposal suggests a DI (drawing and ironing) method be used as a method for manufacturing a battery can in order to increase the productivity of the battery can and reduce the price thereof (for example, see Patent Document 1). In the DI method, a cup-shaped intermediate product is fabricated by means of deep-drawing with a press machine, and then the cup-shaped intermediate product is subjected to ironing and drawing in one continuous stroke, whereby a battery can having a predetermined shape is fabricated. In other words, in the DI method, both drawing and ironing are performed in a single step.
An example of the method of manufacturing a battery can employing the DI method will be hereinafter described.
First, a 0.4 mm thick steel plate is prepared as a base material and the steel plate is heated at 600 to 800° C. for 5 to 20 hours. Subsequently, an Ni-plated layer having a thickness of approximately 3.5 μm is formed on both faces of the steel plate after heat treatment, and then the steel plate with Ni-plated layers is heated at 500 to 650° C. for 1 to 20 hours, thereby to obtain a battery can base material. On each surface of the battery can base material thus obtained, a nickel-iron alloy layer (Ni—Fe alloy layer) is formed. Further, on each of the alloy layers, a nickel layer (Ni layer) is further formed. The Ni—Fe alloy layer is produced mainly by heat treatment that allows Ni atoms to be dispersed into the Fe layer.
The battery can base material is subjected to deep-drawing to be formed into a cup-shaped intermediate product, and then the side portion of the cup-shaped intermediate product is subjected to ironing so that the ratio of the thickness of the bottom portion to the thickness of the side portion (the thickness of the bottom portion/the thickness of the side portion) falls within a range of 1.2 to 3.4, whereby a battery can with a predetermined shape is produced.
In order to preferably perform the DI method, it is necessary to obtain a battery can base material that is free of warpage and uniform in quality. For this purpose, a long-time heat treatment as described above must be performed. In many cases, a box annealing furnace is used for a long-time heat treatment. For example, hoop-shaped steel plate is housed in the box annealing furnace, in which heat treatment is carried out.
As another measure for increasing the productivity of a battery can and reducing the price thereof, there has been another proposal suggested with a focus on the step of heat treatment on a steel plate as a battery can base material (for example, see Patent Document 2). According to this proposal, the use of an ultra-low carbon steel plate having a carbon content of less than 0.009 wt % makes it possible to perform continuous annealing. As a result, the time required for heat treatment is drastically shortened and the productivity of the battery can is improved.
The formation of the Ni-layer on the surface of the inner side of the battery can as descried above makes it possible to obtain an improved corrosion resistance of the battery can.
Moreover, with expansion of application range of batteries and increase in the consumption current of portable equipment, improvement in the high rate discharge characteristics has been increasingly required. In addition, with respect to secondary batteries, improvement in the rapid charge characteristics has been required.
As a measure for improving the rapid charge and discharge characteristics, yet another proposal suggests a nickel-plated steel plate for alkaline manganese batteries, the steel plate being provided with an Ni—Fe alloy layer having an iron exposure rate at the outermost surface of not less than 10% (for example, see Patent Document 3).
In the case where a battery can having an Ni layer on the inner face thereof as suggested in Patent Documents 1 and 2 is used, however, the nickel atoms in the Ni-layer become bonded with oxygen atoms with the passage of time, and a nickel oxide layer extremely low in electrical conductivity is consequently formed. As a result, the internal resistance of the battery is readily increased.
The inner face of the battery can also serving as a terminal of either one electrode of a positive electrode and a negative electrode is normally in contact with the electrode accommodated in the battery can, whereby the battery can and the electrode are electrically connected. Since the battery can and the electrode is in contact via the nickel oxide layer extremely low in electrical conductivity, the internal resistance of the battery is increased.
The battery can formed from the nickel plated steel plate of Patent Document 3 also has a disadvantage in that the rapid charge and discharge characteristics, especially the high rate discharge characteristics, does not sufficiently satisfy a required performance, and needs further improvement. Furthermore, even if the iron exposure rate on the surface of the Ni—Fe alloy layer is not less than 10%, the condition of the inner face of the battery can is easily varied because the nickel or iron on the surface is partially oxidized as the passage of time.    Patent Document 1: Japanese Patent Laid-Open Publication No. Hei 8-55613    Patent Document 2: Japanese Patent Laid-Open Publication No. Hei 6-346150    Patent Document 3: Japanese Patent Laid-Open Publication No. 2002-208382